Early human bipedalism confirmed

John Gurche and Brian Richmond

The shape of a fossil thigh bone (center)
from Kenya shows that our early ancestors were already adapted to upright
walking by 6 million years ago.

Sometime between 5 million and 8 million years ago, an evolutionary
divide occurred: some apes left the trees behind and adopted an upright form
of walking. The two-legged, or bipedal, population formed the basis of the hominin
lineage that gave rise to Homo sapiens and its close ancestors. To find
evidence of the earliest human ancestors, scientists scrutinize fossils from
around this time period.

One such fossil, a 6-million-year-old thigh bone from a species
called Orrorin tugenensis, is the topic of a new study published in Science
yesterday. Despite previous examinations of this fossil, which was discovered
in Kenya in 2000, researchers still debate whether O. tugenensis walked
upright and whether it was more closely related to the australopithecines that
lived 3 million to 2 million years ago or to the much later Homo.

Brian G. Richmond of George Washington University in Washington,
D.C., and William L. Jungers of Stony Brook University in New York write in
Science that they have resolved the debate by determining that O.
tugenensis did indeed walk upright and that its hip structure is more similar
to the australopithecines.

Richmond and Jungers based their conclusions about the species'
bipedalism and hip mechanism on the shape of the thigh bone. Using calipers,
they recorded many of the fossil's dimensions, and then compared their measurements
through statistical analyses to various hominin and ape species.

Determining that O. tugenensis was bipedal is significant,
Richmond says, because it reduces the time window during which the hominin lineage
could have originated to between about 6.5 million to 7 million years ago. The
presence of not only bipedal but also more primitive tree-climbing traits in
the species indicates that O. tugenensis is not far removed from the
ape-human common ancestor but that it is "already in the human line," Richmond
says.

Robert Barry Eckhardt, an evolutionary biologist at Penn
State University in University Park, says the study's findings that O. tugenensis
was bipedal is not new: "[It] is just the same conclusion that my colleagues
concluded on the basis of the very same evidence in 2001." Eckhardt says that
the paper's hypothesis that O. tugenensis is more similar to Australopithecus
afarensis than to modern Homo "may or may not be true," although
he says the statistical data presented within the paper do not suggest that
this is a strong possibility.

Andrew T. Chamberlain, a biological anthropologist at the
University of Sheffield in the United Kingdom, says that the study has quantitatively
and convincingly established bipedalism in the specimen. The findings are significant,
he says. Chamberlain agrees with the researchers that O. tugenensis shared
a hip and gait mechanism with the much younger A. afarensis  such
as the famed Lucy. As the femur "morphology is an early example of australopithecine,"
he says, it demonstrates that the hip mechanism was evolutionarily stable. He
adds that more studies of A. afarensis might confirm this.